Turbojet engines for aircraft are currently equipped with an afterburner device. An afterburner device comprises means for injecting fuel in proximity to flame-holder members and at least one afterburning ignition spark plug located in an afterburning ignition area. Under afterburning conditions, additional fuel is injected in order to obtain an increase in thrust.
Such a turbojet engine thus includes two ignition means: main spark plugs for igniting the main combustion chamber, and afterburning spark plugs for igniting afterburning.
For reasons of simplicity, bulkiness and costs, a single ignition unit simultaneously controls the ignition of the main combustion chamber as well as the afterburning ignition. In this way, independently of whether activation of the ignition of the main chamber or the afterburning is desired, the main spark plugs and the afterburning spark plugs are simultaneously energized. The main spark plugs fire even when only the firing of the afterburning spark plugs is desired.
Now, during the operation of the turbojet engine, a main spark plug is subject to very high pressure, of the order of 30 bars, in altitude. Under these conditions, the firing of a main spark plug implies a rapid degradation of the latter. The lifetime of a main spark plug may be considerably reduced by this, causing additional maintenance costs and reducing the availability of the aircraft as well as the reliability of their performances.
It is therefore desirable to only energize a main spark plug when the latter should be sought, i.e. upon starting or restarting the turbojet engine.
Complete segregation between the routes for igniting a main spark plug and the routes for igniting an afterburning spark plug is capable of meeting this requirement, such that the energization of a main spark plug is independent of the energization of an afterburning spark plug.
However, the application of two ignition units, one dedicated to the main plugs and the other to the afterburning spark plugs, represents an overcost, increased bulkiness and a mass increase which may not be acceptable. Further, the requirement of an additional computer output for the independent controls may be incompatible with the existing systems.
Further, replacement of an ignition unit may not be concomitant with a possible adaptation of other elements of the ignition system, and notably with the adaptation of the control signal sent by the computer for controlling the ignition. Thus, it is necessary that the modifications brought to the ignition system may be applied independently of each other.